The present invention relates to a self-recovery type current limiting element for suppressing an abnormally large current such as a shortcircuiting current in an overcurrent condition and for allowing a normal current to flow under normal conditions before and after the overcurrent condition.
A conventional current limiting element of this type is as shown in FIG. 1. In FIG. 1, numerals 1 and 2 designate first and second current terminals, numeral 3 an electrode, numerals 4 and 5 first and second pistons, numerals 6, 7, 8, 9 and 10 seal rings, numerals 11 and 12 annular insulating members, numeral 13 a special insulator, numeral 14 an outer cylinder, numeral 15 a clamp, and numeral 16 a current limiting material. Numerals 17 and 18 designate buffers forming pressure buffer units of the current limiting material 16 together with pistons 4 and 5, numeral 19 a spacer, numeral 20 an intermediate spacer, numerals 21, 22 and 23 sealers, and numeral 24 an element cylinder.
The first and second current terminals 1 and 2 are formed, for example, of a conductive material such as a chromium copper or a beryllium copper. The terminal 1 is engaged with the electrode 3, and the terminal 2 is engaged with the cylinder 14. Reference character 2a denotes a through hole formed at the terminal 2. The electrode 3 is formed, for example, of a conductive material such as chromium copper or a beryllium copper. Reference character 3a depicts a through hole formed at the electrode 3. The pistons 4 and 5 are respectively provided in the through holes 3a and 2a. The annular members 11 and 12 are formed of an insulating material such as beryllia procelain and alumina porcelain. The outer cylinder 14 is respectively associated with a plurality of annular insulating members 11 and 12 via through holes 11a and 12a connecting the intermediate spacers 20. The current limiting material 16 such as sodium, potassium, NaK formed of a sodium and potassium alloy or mercury (Hg) is provided to fill in through holes 11a and 12a, part of the through hole 3a of the electrode 3 and part of the through hole 2a of the terminal 2. The insulator 13 is formed of solid material produced, for example, by powders of mica and glass and having a thermal expansion coefficient larger than that of the cylinders 11 and 12 and a large mechanical strength such as stainless steel. The clamper 15 prevents the electrode 3 from being removed through the insulator 13 which is filled in the outer cylinder 14 and covers the electrode 3. The element cylinder 24 and the second current terminal 2 are individually formed, and then connectedly associated.
A method of manufacturing the element cylinder 24 is referred to as the so-called "molding". This method includes the steps of first allowing the insulator 13 to be press-fitted over the electrode 3, the insulating cylinders 11 and 12, the intermediate spacer 20, and the clamp 15 and then permitting the spacer 19 to be cooled to the ambient temperature. Thus, radial and axial compression forces due to thermal expansion of the current limiting material 16 filled in the through holes can be applied to the cylinders 11, 12 at differnt strength levels. Another so-called "molding by shrinkage-fitting" method is carried out to form part of a vessel sufficiently durable against high internal pressure. The buffers 17 and 18 are formed of compressive fluid such as argon or nitrogen and a mechanically elastic material such as a coil spring or a leaf spring. The spacers 20 and 19 are formed, for example, of copper or chromium copper, and composed of a material having high thermal conductivity to prevent the insulating cylinders from 11, 12 being damaged by "molding by shrinkage-fitting" method and to improve the heat dissipation. The sealer 21 seals a filling port receiving the material 16. The sealers 22 and 23 respectively seal the filling ports of the buffers 17 and 18.
As shown in FIG. 1, the cross-sectional area of the through holes 11a of the annular members 11 smaller than that of the through holes 12a of annular member 12 os as to satisfy various electrical performance of the current limiting material 16.
The operation of the conventional current limiting element is as follows. As a normal (rated) current flows along a path connecting the first terminal 1 through the electrode 3 and the current limiting material 16 to the second terminal 2, the current limiting material 16 generates Joule heat and changes into the solid or liquid state depending on the temperature of the generated heat and the heat dissipations in radial direction from the members 11 and 12, and in axial direction from the insulator 12.
When an overcurrent such as a shortcircuiting current flows across the current limiting element, the material 16 in the through holes 11a of the member 11 due to its small cross-sectional area is first vaporized. The material 16 in the through holes 12a of the member 12 having a larger cross-sectional area is subsequently vaporized sequentially to become a plasma state of high temperature, pressure and resistance, thereby suppressing (limiting) the overcurrent to a predetermined value or lower. The members 11 and 12 having heat resistance disposed around the material 16 coupled with the insulation from the insulator 13 endure against the high temperature occurred by the plasma state of the material 16 so as to force the pistons 4 and 5 on both sides to move against the high pressure which is absorbed by the compressing operations of the buffers 17 and 18. The members 11, 12 and the insulator 13 further endure against the voltage generated between the current terminals 1 and 2 due to the high resistance of the material 16 when a short occurs.
As can be seen, the current limiting element can only limit the overcurrent by suppressing it to a lower current, but cannot trip the circuit by interruption. However, the element is interrupted by a switch (not shown) provided, for example, in series. After the suppressing operation, the material 16 is then cooled by the heat dissipation, and recovered to the liquid or solid state by the returning pressures from the buffers 17 and 18 via pistons 4 and 5 to allow a normal load current to flow across. In other words, the element has a reenergizing performance.
In the case of FIG. 1, since the vaporization of the current limiting material 16 starts from the insulating member 11 which are in the center and at the farthest distance from the pistons 4 and 5 and then advances to the other portions of the through holes of the members 12, these portions of the through holes of the members 11 and 12 cannot be effectively utilized for the current limiting action.
Further, the pistons 4 and 5 not only operate the pressure buffers 17 and 18 for the reenergizing performance after the overcurrent suppression, but also apply compressing force to the material 16 even when volumetric change occurs due to the variation in the phase from solid to liquid of the material 16 creating air gaps in the cylinders 11 and 12 which will substantially degrade the reenergizing performance of the current limiting element.
Since the conventional current limiting element of the construction described above generally has, however, small thermal conductivity of the special insulator 13, its possible radial heat dissipation amount is small. Thus, the heat dissipation mainly depends upon the axial heat dissipation through the members 11 and 12. In the event that it is necessary to enhance the voltage across the limiting element, the cylinders 11 are positioned between cylinders 12 to further increase the axial length of the element. This allows the material 16 increase its Joule heat generation. It is further noted that as the temperature increases, the reenergizing performance decreases. In addition, the conventional current limiting element has such a drawback that, when the axial length of the members 11 and 12 are increased as described above, long outer cylinder 14 is required, and the manufacture of the cylinder 14 becomes difficult, with the result that the cost increases.